1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, an electrophotographic copying machine or an electrophotographic facsimile receiver.
2. Description of the Related Art
FIG. 5 schematically shows a longitudinal sectional view of a main portion of a conventional image forming apparatus of the above-described type. The main portion comprises a photosensitive drum 10, and further comprises a charger 12, optical image forming means 14, developing means 16, a transfer device 18 and a cleaner 20, all arranged along an outer circumferential surface of the photosensitive drum 10 in a rotating direction of the drum 10 indicated by an arrow A (the clockwise direction in FIG. 5 of the prior art).
In order to form a desired image on a sheet by the image forming apparatus, the photosensitive drum 10 is rotated in a predetermined rotating direction (the clockwise direction indicated by the arrow A in FIG. 5) at a predetermined circumferential speed V0 by a rotational force transmitted from a known rotation driving means (not shown) through a rotational center shaft 10a. During the rotation of the drum 10, the charger 12 (a charge roller abutted to the photosensitive drum 10 and rotated in the same clockwise direction as the rotating direction of the photosensitive drum 10, in this prior art) charges the outer circumferential surface of the photosensitive drum 10 uniformly with a predetermined voltage. The optical image forming means 14 applies light on the electrically charged outer circumferential surface of the photosensitive drum 10 in accordance with a desired image signal supplied to the optical image forming means 14, thus forming an electrostatic latent image corresponding to the desired image on the outer circumferential surface of the drum 10. The developing means 16 includes a development roller 16a abutted to the outer circumferential surface of the photosensitive drum 10. The development roller 16a is rotated at a predetermined circumferential speed V1 which is faster than the circumferential speed V0 of the photosensitive drum 10 (in this prior art, V1=2V0) in an opposite direction (counter-clockwise direction in FIG. 5) to the rotating direction of the photosensitive drum 10, and develops the electrostatic latent image on the outer circumferential surface of the photosensitive drum 10, by using toner T in a toner container 16b. The toner T has only one component of a non-magnetic material. In FIG. 5, the rotational force is transmitted to the development roller 16a by the known rotation driving means (not shown) through a train of gears 16c and 16d.
A paper sheet P supplied from paper sheet supplying means (not shown) to the image forming apparatus is introduced between the photosensitive drum 10 and the transfer device 18 from the right-hand side thereof in FIG. 5 by conveying means (not shown), and a developed toner image is transferred from the outer circumferential surface of the photosensitive drum 10 to the paper sheet P by the transfer device 18. The sheet P on which the toner image has been transferred is conveyed to a fixation device (not shown), situated on a left side of the image forming apparatus in FIG. 5, by the conveying means, and the transferred toner image is fixed on the paper sheet P by the fixation device.
The cleaner 20 (a doctor blade abutted to the photosensitive drum 10 in this prior art) removes toner RT remaining on the outer circumferential surface of the photosensitive drum 10 after the toner image is transferred, and the removed toner RT is stored in a disuse toner container 22.
In the conventional image forming apparatus of the above-described type, in order to improve a quality of the toner image formed on the paper sheet P in the image forming apparatus, it is important that the photosensitive drum 10 is rotated at a constant circumferential speed.
However, to the photosensitive drum 10 rotated in the conventional image forming apparatus having the above-described structure, not only torque generated by the rotational force transmitted to the photosensitive drum 10 from the known rotation driving means (not shown) but also frictional resistance torque generated by frictional force acting on the photosensitive drum 10 are applied. More specifically, the development roller 16a rotated in the direction opposite to the rotating direction of the photosensitive drum 10 in a state in which it is in contact with the outer circumferential surface of the photosensitive drum 10, applies a frictional resistance torque T1 to the photosensitive drum 10 in the same direction as the rotation torque of the photosensitive drum 10. Further, the charge roller of the charger 12 rotated in the same direction as the rotating direction of the photosensitive drum 10 in a state in which it is in contact with the outer circumferential surface of the photosensitive drum 10, the doctor blade of the cleaner 20 fixed to a housing of the image forming apparatus in a state in which it is in contact with the outer circumferential surface of the photosensitive drum 10, and bearings (not shown) for the rotational center shaft 10a, apply a frictional resistance torque T2 to the photosensitive drum 10 in a direction opposite the rotation torque direction of the photosensitive drum 10.
Value of the frictional resistance torque T1 can be obtained by multiplying a frictional force F2 applied to the circumferential surface of the drum 10 by the development roller 16a, by a radius r of the drum 10 (that is, T1=F2.times.r), and the frictional force F2 can be obtained by multiplying an abutting pressure F1 by which the development roller 16a is abutted on the circumferential surface of the photosensitive drum 10, by a kinetic frictional coefficient .mu. acting between the circumferential surface of the photosensitive drum 10 and the development roller 16a (that is, F2=F1.times..mu.; T1=F1.times..mu..times.r).
The value of the frictional resistance torque T2 applied to the photosensitive drum 10 from rotation resistance members other than the development roller 16a is relatively stable, and the rotation resistance members include, for example, the charge roller of the charger 12, the doctor blade of the cleaner 20, the rotational center shaft 10a of the photosensitive drum 10, and the bearings (not shown) for the rotational center shaft 10a of the drum 10. On the other hand, the value of the frictional resistance torque T1 applied to the photosensitive drum 10 from the development roller 16a varies greatly. This is because the circumferential speed V1 of the development roller 16a varies greatly due to variation of the amount of the toner T in the toner container 16b, variation of the value of the kinetic frictional coefficient .mu. acting between the development roller 16a and the photosensitive drum 10, the variation of the value of the kinetic frictional coefficient .mu. being caused by variation of the amount of the toner T introduced between the development roller 16a and the photosensitive drum 10 from the development roller 16a, or the like.
If the value of the frictional resistance torque T1 becomes larger than the value of the frictional resistance torque T2 (T1&gt;T2), the variation of the value of the circumferential speed V1 of the development roller 16a influences greatly the value of the circumferential speed V0 of the photosensitive drum 10.
In order not to make the variation of the value of the circumferential speed V1 of the development roller 16a influence the value of the circumferential speed V0 of the drum 10, in the prior art the abutting pressure F1 of the development roller 16a to the circumferential surface of the photosensitive drum 10 is limited to a low level so that the value of the frictional resistance torque T1 becomes smaller than that of another one, namely, the frictional resistance torque T2. However, if the abutting pressure F1 is lowered under a certain level, the amount of the toner T attached to the circumferential surface of the photosensitive drum 10 by the development roller 16a is reduced, thus lowering printing quality. Consequently, there is a certain limitation for lowering the value of the abutting pressure F1.
The relationship "T1&lt;T2" can be set up by increasing the abutting pressure of the doctor blade of the cleaner 20 to the circumferential surface of the photosensitive drum 10. However, if the abutting pressure of the doctor blade is increased over a certain level, the circumferential surface of the photosensitive drum 10 will be damaged by the doctor blade, and the variation of the abutting pressure of the doctor blade to the circumferential surface of the photosensitive drum 10 caused by the toner TR remaining on the circumferential surface of the drum 10 will be increased. A large variation of the abutting pressure of the doctor blade causes great variation of the circumferential speed of the rotation of the photosensitive drum 10.